Process for producing an infrared detector and associated infrared detector

ABSTRACT

A method of manufacturing an infrared detector includes the steps of: hybrid bonding of a detection chip to a second chip; said hybrid bonding step being carried out by adhesion of contacts and of insulator layers of the two chips; removal of a substrate of said detection chip to reach a deep oxide layer; forming of conductive pads through said deep oxide layer to reach transistors present in a semiconductor layer; and forming of microbolometers suspended over said deep oxide layer and electrically connected to the conductive pads.

DOMAIN OF THE INVENTION

The invention concerns the technical field of so-called “uncooled”infrared imaging, that is, the technical field of infrared detectorscomprising microbolometers suspended over a substrate to decrease theinfluence of the temperature of the substrate and of the environment onthe microbolometer performance.

The invention particularly advantageously applies to decrease thefootprint of an infrared detector comprising a digital processingelectronic system or any additional electronic function requiringincreasing the available surface area, for example, an instantaneousreadout circuit since, in this case, there is a need for additionalsurface area for each pixel of the detector.

BACKGROUND

As illustrated in FIG. 1 of the state of the art, an image detector inthe visible spectrum generally appears in the form of a chip 100, calleddetection chip, which comprises elements 21 photosensitive to visiblelight and a readout circuit conventionally formed in CMOS technology.Photosensitive elements 21 are formed at the junction between a metalinterconnection network 14 and a semiconductor substrate 12.

The readout circuit is formed of transistors formed in semiconductorsubstrate 12 and coupled to metal interconnection network 14. Thereadout circuit includes an analog-to-digital converter which delivers araw digital video signal, onto output contacts 17 of detection chip 100,according to an image captured by photosensitive elements 21. Contacts17 emerge from an oxide layer 11 and are coupled with contacts 18arranged on an electronic board 15 having detection chip 100 bondedthereto. For example, the bonding may be performed by means of glue 20.Further, a microlens 16 is formed on oxide layer 11 to focus the raysonto photosensitive elements 21.

However, in this so-called “top side illumination” configuration, losseson the light flow are relatively significant, since the latter has tocross oxide layer 11 and metal interconnection network 14 beforereaching photosensitive elements 21.

In the visible spectrum range, it is thus desired to flip detection chip100 with respect to the light flow to limit losses.

Further, to improve the quality or the information contained in thedigital video signal, for example, by applying an image correctionand/or by associating a motion detection algorithm, it is known toassociate a digital processing chip with detection chip 100. Similarly,these two chips are conventionally formed in different technologiessince it is often not possible to integrate the digital processingsdirectly in detection chip 100.

For these reasons, several technologies have been developed in thevisible spectrum range to ease the connection of two chips oftechnologies that may be different from each other.

A first solution comprises assembling a first detection chip onto asecond chip, for example, a digital processing chip, this second chipbeing itself assembled on an electronic board. The contacts of thesecond chip are then arranged around the location intended to receivethe detection chip, and connections, for example, wire connections, areformed between the contacts of the second chip and the contacts of thedetection chip and between the contacts of the second chip and thecontacts of the electronic board. This solution has the defect of widelyincreasing the footprint and of generating noise or latencies in thecommunication between the two chips.

To limit this problem by suppressing the contacts around the detectionchip, a second solution provides forming connection paths through thesemiconductor substrate of the detection chip to form contacts under thedetection chip to the processing chip.

However, the paths have to be positioned around the area occupied by thephotosensitive elements, which still causes an increase in the footprintof the detection chip. Based on these technical solutions, severalimplementation variants are possible, for example, two chips may beassembled on two opposite surfaces of a same electronic chip by theconnection path technique.

These two solutions are the only solutions implemented in the field ofuncooled infrared imaging to associate the detection chip with anotherchip, for example a digital processing chip. In this specific technicalfield, the photosensitive elements correspond to microbolometerssuspended over an upper layer of the detection chip to decrease theinfluence of the temperature of the substrate and of the environment onthe microbolometers. To form these suspended microbolometers, theconventional technique comprises using at least one sacrificial layerdeposited at the surface of the detection chip, and where openings arecreated to allow the deposition of conductive pads enabling to maintainthe microbolometers suspended. On this sacrificial layer, the structureof the microbolometers is then formed so that the conductive pads cansupport the microbolometers during the removal of the sacrificial layer.

In visible imaging, it is known to form a face-to-face bonding betweenthe two chips to further limit the footprint. The photosensitiveelements are then illuminated through the semiconductor substrate of thedetection chip, once it has been thinned. This configuration is called“bottom side illumination”.

In the example of FIG. 2 of the state of the art, a first detection chip100 is bonded to a second chip 105 comprising transistors 24 allowingadditional readout or digital processing functions carried out in asolid substrate 22. Preferably, the bonding between the two chips 100,105 is obtained by a “mixed bonding”, that is, an adhesion between metalareas 17, 23 on the one hand and between oxide layers 11, 46 on theother hand. For example, the adhesion of metal areas 17, 23 may beobtained by thermocompression and the adhesion of oxide layers 11, 46 bymolecular bonding.

This technique is known as “hybrid bonding”.

Detection chip 100 is flipped and insulating layer 11 is arranged undersemiconductor substrate 12. Contacts 17 flush with insulating layer 11are positioned under detection chip 100 and they are placed oppositecontacts 23 of second chip 105 during the hybrid bonding of the twochips 100, 105.

Semiconductor substrate 12 is then thinned to limit optical losses andoptimize the illumination of photosensitive elements 21. Theillumination is also controlled by a microlens 16 formed onsemiconductor layer 12. The connections between electronic board 15 andsecond chip 105 are, for example, formed by through connection paths 26emerging onto metallizations 25, and connected to contacts 18 ofelectronic board 15, arranged under second chip 105.

As a result, the footprint of the embodiment of FIG. 2 is smaller at thesurface of electronic board 15 than the footprint of the embodiment ofFIG. 1, while the embodiment of FIG. 2 integrates a second chip 105integrating readout or digital processing functions. The estimatedsurface gain on the electronic board is in the order of from 30 to 40%.

However, this embodiment of FIG. 2 imposes a back-side illumination ofphotosensitive elements 21 since detection chip 100 is flipped.

Given the constraints of the manufacturing of the microbolometerssuspended over the upper surface of the detection chip, it seemsimpossible to directly repeat this embodiment of FIG. 2 in the field ofuncooled infrared imaging.

The technical problem of the present invention is to decrease thefootprint of a detection chip associated with a second chip integratingadditional readout or digital processing functions, in the field ofuncooled infrared imaging, that is, for a detection chip comprisingmicrobolometers suspended over an upper layer of the detection chip.

SUMMARY

To respond to this problem, the invention provides using a detectionchip comprising a fully depleted semiconductor layer integratingtransistors and other active elements allowing the readout function,before transferring the detection chip onto a second chip and formingthe suspended microbolometers after the hybridization of the two chipstogether.

In the sense of the invention, a chip comprising a fully depletedsemiconductor layer corresponds to a FDSOI-type (Fully Depleted SiliconOn Insulator) CMOS technology This chip natively comprises a thin layerof insulator and a thin semiconductor layer interposed between thesubstrate and the metal interconnects of the CMOS circuit. The thininsulator layer is called deep oxide layer, and the thin semiconductorlayer is called “silicon on insulator” (SOI) although othersemiconductor materials may be used, such as germanium or galliumarsenide. This chip thus natively appears with a substrate, topped witha deep oxide layer, and then a thin semiconductor layer wheretransistors and other active elements are formed, and a metalinterconnection network ending at the surface by an insulating layerthrough which electric contacts may emerge. The fineness of thesemiconductor layer enables to obtain a circulation of charges acrossthe entire thickness of the semiconductor layer.

For this purpose, according to a first aspect, the invention concerns amethod of forming an infrared detector comprising the steps of:

hybrid bonding of a detection chip onto a second chip; and during thehybrid bonding;

said detection chip comprising a substrate topped with a deep oxidelayer, a fully depleted semiconductor layer integrating transistors, ametal interconnection network, and an insulator layer;

said detection chip comprising a hybridization surface having contactsemerging from said insulator layer and connected to the metalinterconnection network; and

said second chip comprising a substrate having transistors and contactsemerging from an insulating layer at the level of a hybridizationsurface formed therein;

the hybrid bonding step being carried out by adhesion of the contactsand of the insulator layers of the two chips;

suppression of the substrate of said detection chip to reach said deepoxide layer;

forming of conductive pads through said deep oxide layer to reach thetransistors present in said semiconductor layer; and

forming of suspended microbolometers over said deep oxide layer andelectrically connected to the conductive pads.

The invention thus enables to obtain an infrared detector integratingremote analog functions or advanced digital functions, formed by thesecond chip, while having a small surface area on an electronic board.It is thus possible to provide infrared detectors integrating complexfunctions at a lower production cost and having a very high compactness.The invention describes the use of transistors in the chips. Of course,these transistors may be completed with other active elements,particularly of diode type.

Further, the microbolometers may have structures equivalent to thecurrent structures, so that the performance of the infrared detector isnot decreased.

To increase the performance of microbolometers, the method may alsocomprise a step of forming a metal reflector on said deep oxide layer.This reflector results in sending back part of the thermal energyarriving onto the deep oxide layer towards the bolometric membrane ofeach bolometer, thus creating a so-called Fabry-Perot resonant cavity.

According to a specific provision of the invention, the method alsocomprises a step of contact forming through said deep oxide layer toreach an area of said semiconductor layer connected to the metalinterconnection network, said metal reflector being formed on saidcontacts to form an electrode or a ground plane. Indeed, all thereflectors or electrodes may for example be coupled together to form aground plane facing the transistors present in the semiconductor layerof the readout circuit.

Further, the microbolometers may be formed with known and tried methods,typically by using conductive pads formed in a sacrificial layer, and byforming a detection membrane on the sacrificial layer and connected tothe conductive pads.

According to an embodiment, said semiconductor layer being divided, inthe footprint of each pixel, into a plurality of distinct areas formingan injection transistor, the microbolometer forming step is carried outto connect in series each microbolometer onto an injection transistor.This injection transistor has the function of generating a controlledelectric voltage on said microbolometer. Preferably, the distinct areasof said semiconductor layer form at least the poles of a MOSFET-typetransistor.

Preferably, said hybrid bonding step is carried out by thermocompressionof the contacts and by molecular bonding of the insulator layers of thetwo chips.

According to a second aspect, the invention concerns an infrareddetector comprising:

a detection chip comprising transistors connected on the one hand tomicrobolometers suspended over a deep oxide layer and, on the otherhand, through a metal interconnection network, to contacts emerging froman insulator layer; and

a second chip integrating transistors connected to contacts emergingfrom an insulator layer;

the contacts and said insulator layer of said detection chip beingbonded to the contacts and said insulator layer of said second chip.

The transistors of the second chip may have various functions and may becompleted by other active elements, such as diodes. For example, thetransistors complete those of the first chip to form the readoutcircuit, to support remote functions or to perform a digital processingof the signal originating from the detection chip. The complementarityof the transistors of the two chips enables to obtain processingfunctions applied pixel by pixel and no longer only generally, which isimpossible with other conventional transfer techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

The way to implement the present invention, as well as the resultingadvantages, will better appear from the description of the followingnon-limiting embodiment, given as an indication, based on theaccompanying drawings, among which FIGS. 1 and 8 show:

FIG. 1: a cross-section view of a detection chip directly assembled onan electronic board according to the state of the art;

FIG. 2: a cross-section view of a chip of detection in the visible rangeassembled on a second chip according to the state of the art;

FIG. 3: a cross-section view of an infrared detector according to anembodiment of the invention;

FIG. 4: a local top view of a detection element of the infrareddetection element of FIG. 3 (FIG. 4b ) showing an injection transistorand a microbolometer connected in series, as well as partialcross-section views along axis AA′ (FIG. 4a ) and axis BB′ (FIG. 4c );

FIG. 5: a cross-section view of a step of the manufacturing of adetection chip (FIG. 5a ) and of a second chip (FIG. 5b ) of theinfrared detector of FIG. 3;

FIG. 6: a cross-section view of a step of hybrid bonding of the chips ofFIG. 5;

FIG. 7: a cross-section view of a step of forming of pads on thedetection chip of FIG. 6; and

FIG. 8: a cross-section view of a step of forming of microbolometers onthe pads of FIG. 7.

DETAILED DESCRIPTION

The present specification is described hereafter in reference to thehybridization of a single detection chip on a single second chip, this,for understanding and illustration simplicity reasons. In practice, theinvention will most often be implemented to simultaneously bond a waferintegrating a plurality of detection chips to a wafer integrating aplurality of second chips.

FIG. 3 illustrates a cross-section view of an infrared detector 9according to an embodiment of the invention. This infrared detector 9comprises two distinct stacked chips to decrease the surface area of useof infrared detector 9 on an electronic board 15.

For this purpose, a detection chip 10 is assembled on a second chip 13,itself assembled on electronic board 15. The connections betweenelectronic board 15 and second chip 13 are formed by connection paths 26on a metal interconnection network 25 of second chip 13 and connected tocontacts 18 of electronic board 15. Thus, by using connection paths 26,it is possible to connect second chip 13 via contacts 18 arranged undersecond chip 13. In practice, solder balls are conventionally arranged oncontacts 18 to ensure an electric contact and a holding betweenconnection paths 26 and contacts 18. As a variant, other connectiontypes may be used between second chip 13 and electronic board 15.

This second chip 13 comprises transistors 24 that may be used with otheractive elements, such as diodes. For example, these transistors 24enable to carry out different functions that cannot be integrated in thesurface of the pixel of the detection circuit: simultaneous reading fromthe “snapshot”-type detection elements, or analog-to-digital conversionat the pixel level, or a digital processing of the signal originatingfrom detection chip 10. As a variant, any other processing may beimplemented by transistors 24 without changing the invention.

This second chip 13 also has contacts 23 emerging from an upper surfaceof the second chip 13 and connected to transistors 24. The upper surfaceof this second chip 13 corresponds to a hybridization surface 31 whichis opposite to the surface of this second chip 13 intended to come intocontact with electronic board 15.

In the sense of the specification, the formulation according to whichthe contacts “emerge” from a surface indicates that the contacts arecoplanar with a terminal end of the chip at the level of the surface.However, in the drawings, the fineness of these contacts does not enableto represent them otherwise than schematically.

Detection chip 10 is bonded to this second chip 13 at the level ofcontacts 17 emerging from a hybridization surface 30 of detection chip10. For this purpose, a hybrid bonding is performed so thathybridization surfaces 30 and 31 are planarized and contacts 17 and 23are embedded in a silicon oxide matrix. A molecular bonding is obtainedby hybrid bonding between the silicon oxide surfaces, while contacts 17and 23, typically made of copper, are soldered by thermocompression.

As illustrated in FIGS. 5a and 5b , the elements forming detection chip10 and second chip 13 are preferably formed before the hybrid bonding ofdetection chip 10 onto second chip 13. For this purpose, detection chip10 is formed on a substrate 80 topped with a deep oxide layer 45, andthen a fully depleted thin semiconductor layer 42 where transistors areformed, and a metal interconnection network 61 ending at the surfacewith an insulator layer 11 through which electric contacts 17 mayemerge.

During the hybrid bonding, detection chip 10 is flipped so that contacts17 are now arranged at the level of the lower portion of detection chip10. After having matched the contacts 17 of detection chip 10 with thecontacts 23 of second chip 13, it is possible for detection chip 10 tobe slightly offset with respect to second chip 13, as illustrated inFIGS. 3 and 6 to 8.

Of course, the illustrated offset is exaggerated and only aims atillustrating the transfer which has occurred between the two chips 10and 13. After the transfer of detection chip 10 onto second chip 13, thesubstrate 80 of detection chip 10 may be removed by physical or chemicalprocessing to reach deep oxide layer 45.

Openings are then formed through deep oxide layer 45 to reach thetransistors 40 of the semiconductor layer 42 of detection chip 10. Toobtain a satisfactory electric continuity, an ohmic contact may beformed by siliciding on the surface of semiconductor layer 42 at thelevel of the previously-formed openings. To form each microbolometer 41,it is then possible to form two conductive pads 50 a and 50 b through asacrificial layer deposited on deep oxide layer 45, so that the currentis injected into microbolometer 41 through pad 50 a and that the currentreturns to transistors 40 through pad 50 b. Conductive pads 50 arearranged vertically in line with the previously-formed contacts.

Preferably, conductive pads 50 are arranged on specific areas ofsemiconductor layer 42, may highly conductive by a strong local doping,for example N++. This processing is performed well upstream of themanufacturing of the CMOS circuit by conventionally using implantationmethods. Thus, each pixel of infrared detector 9 is directly formed on aMOSFET-type (Metal Oxide Semiconductor Field Effect Transistor)transistor and appropriately connected thereto to provide a controlledbiasing to each microbolometer.

To form this MOSFET transistor, semiconductor layer 42 is for examplestructured with an N++-doped area 53 forming the drain of the MOSFETtransistor, stacked to a lightly P-type doped area 58 forming thechannel of the MOSFET transistor and also stacked to a second N++-dopedarea 57 forming the source of the MOSFET transistor. The looping back ofthe current after its flowing through the microbolometer is ensured byan N++-doped area 55, as illustrated in FIGS. 4a -4 c.

The previously-described transistor is a PMOS-type transistor since thechannel is P-type doped. Of course, this is an example only and theinvention may also be formed on an NMOS-type transistor by inverting thedoping types. Further, the space occupation described in FIGS. 4a-4c isa very simple example only. In reality, and especially for small pixels,the injection transistor is common to a group of 2 or 4 pixels, and itis surrounded with many other much smaller transistors which fulfill thefunction of switches.

Preferably, on manufacturing of the transistors on detection chip 10,trenches are formed in semiconductor layer 42 to delimit the perimeterof each MOSFET transistor or of the areas dedicated to the flowing ofthe current to the bolometer, after which these openings are filled bythe deposition of an insulating material. This enables to insulate eachtransistor from electric disturbances originating from the neighboringareas.

Thus, as illustrated in cross-section BB′, the structure ofsemiconductor layer 42 along a transistor thus comprises an insulatorarea 52, an N++-doped area 53, a P-doped area 58, an N++-doped area 57,and an insulator area 52. Cross-section AA′ is formed of an N++-dopedindependent area 55 allowing the looping back of the current after itsflowing through the bolometer, of an undoped and unused area 54, and ofa second N++-doped area 59. The three areas 54, 59, and 55 are insulatedby trenches filled with an insulating material 52.

Each microbolometer 41 is formed to be suspended from a conductive pad50 a connected on the drain 53 of the MOSFET transistor to reach theconductive pad 50 b connected to the N++-doped area 55 forming thelooping back of the current towards the circuit. P-type doped area 58,which corresponds to the transistor channel, is arranged between the twoN++-doped areas 53 and 57 which respectively correspond to the drain andto the source of said transistor. The gate 44 of said transistor iscoupled to the interconnection network 61 of the readout circuit of thedetection chip, so that a controlled electric voltage may be appliedthereto. Thus, said transistor enables to adjust the biasing applied tothe adjoining bolometer. The transistors are then coupled by pads 62 tometal interconnection network 61.

Preferably, to improve the detection properties of the microbolometer 41formed on pads 50 a and 50 b, it is possible to cover the surface ofdeep oxide 45, on an area corresponding to gate 58, with a reflector 60.According to a specific provision of the invention, this reflector 60may also be connected to the N++-doped area 59 of semiconductor layer42, the latter being itself connected to the internal interconnectionnetwork 61 of the readout circuit of the detection chip. Thus, reflector60 also enables to apply to the surface opposite to the transistor gate,an electric field on the channel of the MOSFET transistor through deepoxide 45. This device enables to confine the charges which flow througharea 58 at the center of semiconductor layer 42.

As illustrated in FIG. 7, reflector 60 may be deposited on deep oxidelayer 45 before the forming of conductive pads 50. Thus, after thebonding of detection chip 10 to second chip 13 by hybrid bonding andthen removal of the substrate of chip 10, the embodiment may comprisethe steps of:

creation of openings in deep oxide layer 45 above areas 53 and 55 toreceive conductive pads 50 a and 50 b, and above area 59 to connectreflector 60;

forming of ohmic contacts in the openings;

forming of reflector 60 on deep oxide layer 45;

deposition of sacrificial layer 70;

forming of conductive pads 50 a, 50 b; and

forming of microbolometers 41.

It is also possible to protect the ohmic contacts above areas 53 and 55during the forming of reflector 60 by forming these contacts after theforming of reflector 60. Thus, the embodiment may comprise the steps of:

creation of an opening in deep oxide layer 45 above area 59 to connectreflector 60;

forming of ohmic contacts in the opening;

forming of reflector 60 on deep oxide layer 45;

creation of openings in deep oxide layer 45 above areas 53 and 55 toreceive conductive pads 50 a and 50 b;

forming of ohmic contacts in the openings;

deposition of sacrificial layer 70;

forming of conductive pads 50 a, 50 b; and

forming of microbolometers 41.

The invention thus enables to obtain an infrared detector 9 using asmall surface area on an electronic board 15 while integrating advancedfunctions, such as the simultaneous reading from the “snapshot”-typedetection elements, or analog-to-digital conversion at the level of thepixel, or digital processings.

The invention claimed is:
 1. A method of manufacturing an infrareddetector comprising the steps of: hybrid bonding of a detection chiponto a second chip, and during the hybrid bonding; said detection chipcomprising a substrate topped with a deep oxide layer, a fully depletedsemiconductor layer integrating transistors, a metal interconnectionnetwork, and an insulator layer; said detection chip comprising ahybridization surface having contacts emerging from said insulatorlayer, and connected to the metal interconnection network; and saidsecond chip comprising a substrate having transistors and contactsemerging from an insulator layer at the level of a hybridization surfaceformed therein; the hybrid bonding step being formed by adhesion of thecontacts and of the insulator layers of the two chips; removal of thesubstrate of said detection chip to reach said deep oxide layer; formingof conductive pads through said deep oxide layer to reach thetransistors present in said semiconductor layer; and forming ofmicrobolometers suspended over said deep oxide layer and electricallyconnected to the conductive pads.
 2. A method of manufacturing aninfrared detector according to claim 1, wherein the method alsocomprises a step of forming a metal reflector on said deep oxide layer.3. A method of manufacturing an infrared detector according to claim 2,wherein the method also comprises a step of forming of contacts throughsaid deep oxide layer to reach an area of said semiconductor layerconnected to the metal interconnection network, said metal reflectorbeing formed on said contacts to form an electrode or a ground plane. 4.A method of manufacturing an infrared detector according to claim 1,wherein said semiconductor layer is divided, in the footprint of eachpixel into a plurality of distinct areas forming an injectiontransistor, the step of forming microbolometers being carried out toconnect in series each microbolometer onto an injection transistor.
 5. Amethod of manufacturing an infrared detector according to claim 4,wherein the distinct areas of said semiconductor layer form aMOSFET-type transistor.
 6. A method of manufacturing an infrareddetector according to claim 1, wherein the step of forming conductivepads and the step of forming microbolometers suspended over said deepoxide layer are carried out by the use of at least one sacrificiallayer.
 7. A method of manufacturing an infrared detector according toclaim 1, wherein said hybrid bonding step is carried out bythermocompression of the contacts and by molecular bonding of theinsulator layers of the two chips.
 8. An infrared detector comprising: adetection chip comprising transistors, connected on the one hand tomicrobolometers suspended over a deep oxide layer, and connected on theother hand, through a metal interconnection network, to contactsemerging from an insulator layer; and a second chip integratingtransistors connected to contacts emerging from an insulator layer;wherein the contacts and the insulator layer of said detection chip arebonded to the contacts and the insulator layer of said second chip. 9.An infrared detector according to claim 8, wherein the transistors ofsaid second chip complete the transistors of said detection chip to forma circuit for reading from the microbolometers.
 10. An infrared detectoraccording to claim 8, wherein the transistors of said second chipperform a digital processing of a signal originating from said detectionchip.